1. Field of the Invention
This invention relates generally to the area of fluid flow. In greater particularity the invention relates to controlling the flow of gas injected into a liquid.
2. Description of the Related Art
Gaseous jets submerged in liquids are a feature of many industrial processes such as those, for example, in the chemical, metal and food processing industries as well as the power generation industry. Of particular importance to the United States Navy is liquid metal combustion involving the injection of a gaseous oxidant such as sulfur hexafluoride into a liquid metal fuel such as lithium. The Navy uses liquid metal combustion for compact, self-contained power generators such as those employed in marine propulsion systems. The Stored Chemical Energy Propulsion System (SCEPS) is an example of such a propulsion system.
In liquid metal combustion reactors, it is known that under certain operating conditions erosion of the oxidant injector can occur. Jet instabilities, most likely caused by nonuniform or unstable entrainment of fuel into the jet, permit the injector to be contacted by the corrosive reactants as well as the high temperature liquid metal combustion itself. The injector may eventually be destroyed, resulting in a breach of its containment vessel with loss of power from the propulsion system or even an explosion if liquid lithium contacts water.
In normal operation an oxidant injector is "choked" so that the oxidant gas exits the injector at the speed of sound. Initially, as the oxidant gas passes through the injector passage it is at a low temperature and undissociated, i.e. not very reactive. The high speed of the jet and the low temperature of the gas causes most of the reaction to take place away from the injector. Under these conditions there is little or no attack of the injector.
However, when injecting low-density gas into much higher density liquid, an instability occurs. The instability produces a large-scale disturbance in the jet and causes the injector to be directly exposed to the combustion. The gas becomes partially mixed with the high temperature molten metal bath and reacts rapidly with the injector to cause major injector erosion.
This instability, or what has become known as a "reverse shock" instability of gaseous jets submerged in liquids, was described by the Soviets in 1983. No technique, however, was proposed for its elimination. The Surin article referenced above describes these early observations. A similar instability has been observed by Loth and Faeth in submerged air jets in water. An attempt was made to eliminate this instability, but no success was reported. The Loth and Faeth work is described in their article referenced above.
Due to similarities between non-condensing/non-reacting jets and the reacting jet characteristics of metal combustion, the Navy has investigated nitrogen jets submerged in water. For comparison to rapidly condensing systems, studies of steam injected into water were made.
Both non-reacting and reacting systems are considered vulnerable to the reverse flow effect. Cho studied HCl gas injected into an aqueous solution of NH.sub.3 as a visualizable model of a liquid metal combustion process. He observed an effect similar to the reverse flow effect. These observations are recorded in his article incorporated above.
The reverse flow effect occurs throughout a wide range of operating conditions. Two conditions necessary for its existence are a large difference in density between an injected gas and a liquid bath and that the gas jet not be rapidly condensed into the bath. Both of these conditions occur in liquid metal combustion. This is reported in the Ogden article incorporated by reference herein.
In FIG. 1 there is shown a strobed videoframe of a gaseous jet 1 injected into a liquid 2 under choked conditions. This image illustrates the undisturbed condition of gaseous nitrogen as it is injected into water. As can be seen, jet 1 makes a clean exit from injector 3 posing only minimal exposure of the injector to the injected gas.
In FIG. 2 a strobed video image is shown of the onset of reverse flow. The reverse flow phenomenon is characterized by a sudden reversal of gas flow from the direction of injection. In FIG. 2, a large amount of liquid is entrained into the gaseous jet at 4, blocking the free flow of gas jet 1 in the direction of injection. Because the entrained liquid is many times more dense than the injected gas, a large volume of gas collects at 6 behind the heavy liquid barrier at 4 and is channeled back towards injector 3.
Under these conditions, the injected gas begins to envelop the injector exit and as time progresses a more complete envelopment of the injector will occur. A pressure wave accompanies the reverse flow and results in high intensity noise.